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Summary

Other than being an excellent source of protein, vitamins A and D as well as minerals such as selenium, fish and other seafood are particularly rich in a type of fat which has been found to have significantly beneficial health effects. The term “Omaga-3” has become well known in recent years, due to its purported health benefits. “Omega-3” simply refers to polyunsaturated fatty acids (long hydrocarbon chains with two or more double bonds) whose first double bond occurs at the third carbon from the end of the chain. This may sound a bit complicated to those without a background in organic chemistry, so perhaps it is more important to explain which foods these fats are likely to be found in.The most common plant source of Omega-3 fatty acids is the 18-carbon long “alpha-linolenic acid” ( ALA ). This type of Omega-3 is particularly rich in plant sources such as flaxseed (linseed) oils, walnuts and other plant-based fats. Whilst ALA is thought to have some beneficial health effects, its main role in the body is to help create the longer chain Omega-3 fatty acids DHA (20-carbon long) and EPA (22-carbon long).

Marine foods (fish/seafood), particularly fatty fish such as salmon, are especially rich sources of DHA and EPA. Consumption of fish has been shown to have cardiovascular health benefits such as reductions in serum triglycerides (a type of fat in the blood known to increase the risk of heart disease) as well as reductions in the risk of heart attacks and strokes (cardiovascular disease).

In addition to its protective effect on the cardiovascular system, fish is often referred to as 'brain food'. This is because preliminary studies suggest that higher consumption of fish is linked with a decrease in the risk of cognitive decline and dementia, a decrease in the risk of depression as well as better cognitive / mental development in children.

Higher consumption of fish is may also be associated with a reduced risk of hormonally related cancers such as prostate and breast cancer. This may be due to a combination of its rich selenium (antioxidant mineral) and vitamin D (hormonally related vitamin) content, as well as its EPA and DHA fatty acids; though ultimately, the protective mechanisms are poorly understood.

Fish oil supplementation has shown some success in decreasing the risk of heart attacks, and some studies suggest that it may also help with behavioral problems in children and have an anti-inflammatory effect in those with rheumatoid arthritis and maybe even asthma. Fish oil supplements however are generally poor sources of some of the other important nutrients found in fish, which is why supplements are generally a poor substitute for the real thing.

Fish that are higher up the food chain such as shark and swordfish tend to accumulate mercury, which is why caution should be taken in which fish to eat and which to avoid; especially important during pregnancy. Fish such as salmon however are not only low in mercury, they are one of the richest sources of Omega 3 fatty acids

Discussed below is a review of some of the evidence supporting the recommendations for frequent consumption of fish and seafood, specifically in relation to the Omega-3 fatty acid content.

Fish & Coronary Heart Disease

Epidemiology

Several epidemiological studies have shown that fish consumption can significantly reduce the risk of death from coronary heart disease (CHD) among men and women.

A cohort study in the Netherlands followed 852 middle-aged men for 20 years during which time, 78 had died from Coronary Heart Disease (CHD). Examination of the men’s dietary history revealed that death from CHD was more than 50 per cent lower among those who consumed at least 30 g of fish per day than among those who did not eat fish. [ 1]

Another cohort study in the Netherlands, this time following elderly men for 17 years, found an inverse relation between fish consumption and death from CHD. The rate of death from CHD among those that ate fish was 50% lower than those that did not. [ 2]

The Chicago Western Electric Study involved 1822 men who were 40 to 55 years old and free of cardiovascular disease at the beginning of the study. [3] These men were followed for 30 years after which time 430 had died from CHD, 293 of them from myocardial infarction (heart attack). Analysis of the men’s dietary intakes found that death from CHD was 38% lower among men who consumed 35 g or more of fish daily as compared with those who consumed none. Fish consumption offered even more protection against myocardial infarction as deaths from heart attacks were 67% lower among fish consumers.

A prospective study in China followed 18,244 men aged 45-64 for 10 years after which time 133 had died of acute myocardial infarction (sudden heart attack). Analyses found that death from myocardial infarction was 59% lower among the men who consumed 200 grams or more of fish/shellfish per week. [4]

The Nurses' Health Study followed 84 688 female nurses aged 34 to 59 years who filled out dietary surveys every 4 years for 16 years during which time 1513 cases of CHD occurred, 484 of which were fatal. [5] Compared with those who rarely ever ate fish, the incidence of CHD was 21% lower among women who consumed fish 1 – 3 times per month, 29% lower among those who consumed fish once per week, 31% lower among those who ate fish 2 – 4 times per week, and 34% lower among those consuming fish 5 or more times per week. Once again, this study found that fish consumption had an even stronger protective effect on death from myocardial infarction, as those who consumed fish 5 or more times per week had 45% less fatal heart attacks and 27% less non-fatal heart attacks.

The Health Professionals Follow-up Study followed 44,895 male health professionals, 40 to 75 years of age for 6 years during which time 1543 coronary events occurred, 264 of which resulted in death. [6] Although fish consumption did not appear to be associated with a decreased risk of CHD incidence, compared to men who did not consume fish at all, coronary deaths were 26% lower among men who consumed any amount of fish.

The Seven Countries Study which followed 12 763 middle-aged men over 25 years found that fish intake was inversely related to death from CHD however this association became less significant after statistical adjustment for various confounding factors. [7]

Results from experimental studies in animals suggest that recent dietary intake of long-chain Omega-3 polyunsaturated fatty acids (PUFAs), compared with saturated and monounsaturated fats, reduces vulnerability to ventricular fibrillation, a life-threatening cardiac arrhythmia that is a major cause of heart disease mortality.

The US Physicians' Health Study followed 20, 551 healthy male physicians 40 to 84 years of age for 11 years. There were 133 sudden cardiac deaths (death within 1 hour of symptom onset) over the course of the study. The results found that compared with men who consumed fish less than monthly, those who ate fish at least once per week had 52% less deaths from sudden heart attack. [8]

A case control study in Washington compared the diets of the 493 people who had suffered a heart attack and 493 age-matched controls (persons of a similar age who had never had a heart attack). [9] The diet analysis focused on what the subjects had eaten 1 month prior to their heart attack. This study also collected blood samples given by the healthy controls as well as those collected by the paramedics attending the heart attack patients. The purpose of this was to assess the dietary intake of long chain Omega 3 fatty acids (the beneficial fat found in fish/seafood) by examining the fatty acid composition of red blood cell membranes. The results found that both the average fish intake and blood cell membrane concentrations of long-chain Omega 3 fatty acids (EPA and DHA) was lower in the heart attack patients than what it was in the healthy controls; in fact, an inverse association was observed. Compared with no seafood intake, 1 serving per week was associated with a 50% reduction in the risk of primary cardiac arrest. Moreover, compared with an Omega 3 concentration of 3.3% of total fatty acids (the average concentration found in the lowest consumers of fish), a red blood cell membrane concentration of 5.0% (the average concentration in moderate fish consumers) was associated with a 70% reduction in the risk of primary cardiac arrest.

Another case-control study compared the fatty-acid composition of blood samples given by 94 men in whom sudden death occurred as the first manifestation of cardiovascular disease and for 184 men of the same age who had never suffered a heart attack. [10] As in the previously mentioned study, the blood concentrations of EPA and DHA (the Omega-3 fatty acids found in fish) were also inversely related to the risk of sudden cardiac death. Compared with men with the lowest blood levels, there were 81% less sudden cardiac deaths among those with the highest levels of these Omega-3 fatty acids.

Intervention Trials

Several randomized controlled intervention trials have found that both fish consumption and fish oil supplementation may decrease the risk of subsequent heart disease incidence among people with a prior history of CHD.

In one study, 2,033 men who had suffered non-fatal heart attacks were either given no dietary intervention (controls) or told to either increase cereal fibre intake, decrease total fat intake, or increase fatty fish intake. [11] Two years later, the men who had increased their intake of fatty fish by 200-400g per week had a 29% decrease in subsequent fatal heart attacks than any of the other dietary interventions and controls.

In another intervention trial, patients who had suffered a non-fatal heart attack were randomly assigned to be given either fish oil capsules (1.8 g/day of EPA + DHA), mustard oil (containing 2.8g/day of alpha linolenic acid) or a placebo for 1 year. [12] By the end of the trial, 22% of the patients given the placebo had died of CHD, whereas only 11.4% of those given fish oil had died of CHD. Total cardiac deaths showed no significant reduction in the mustard oil group. Moreover, after 1 year, 25.4% of patients given the placebo had suffered another non-fatal heart attack, whereas only 13% given the fish oil and 15% given the mustard oil had.

In the largest controlled trial examining the effect of fish oil on heart disease, a total of 11, 323 CHD patients who had suffered a non-fatal heart attack within the last 3 months, were randomly assigned to either receive an Omega 3 supplement (850 mg/day of EPA + DHA), a vitamin E (300 mg/d) supplement, both, or no treatment (controls). [13] After only 4 months, those receiving the Omega 3 supplement had a risk of sudden death 53% lower than those not given Omega 3 supplements. This protective effect continued throughout the duration of the trial (3.5 years) during which time a total of 1031 deaths occurred. Those taking the Omega 3 supplement had a significantly lower risk of sudden death, death from CHD, death from Cardiovascular Disease, and death from all causes. Whilst the Omega 3 treated group had an improvement in serum lipid profile (TC, LDL, HDL, TG) the most significant effect was a decrease in triglycerides (TG).

In 2002, researchers published a meta-analysis of 11 randomized controlled intervention trials involving a total of 7,951 CHD patients in the intervention groups given either fish or fish oil supplements and 7,855 patients in the control groups. [14] Two of these trials involved dietary intervention, the other 9 involved fish oil supplements (0.3 – 0.6 g EPA and 0.6 – 3.6g DHA per day). The average follow up lasted 20 months. The average results found that compared with patients assigned a control diet or placebo, those given fish or fish oil supplements had a 20% reduction in non-fatal heart attacks, 30% reduction in fatal heart attacks, 30% reduction in sudden death, and 20% reduction in all causes of death.

Mechanisms

Whilst these studies provide sufficient evidence to conclude that fish consumption and fish oil supplementation my improve the odds of avoiding both fatal and non fatal heart attacks, the mechanisms by which this beneficial effect occurs has been the topic of further investigation. Atherosclerosis is the name given to the condition where plaque build up occurs on the inside of arteries, causing them to narrow and lose their elasticity (harden). When this narrowing and hardening occurs in the arteries which supply the heart muscle with blood (the coronary arteries) or those that supply the brain with blood (cerebral arteries), blood clots can form at these sites and the surrounding tissue dies due to lack of oxygen rich blood supply. When this occurs in the coronary arteries, the result is known as “myocardial infarction” (heart attack), and when it occurs in the cerebral arteries, the results is called a cerebral infarction (ischemic stroke). It has been suggested that fish and fish oil may inhibit the development of atherosclerosis or even cause atherosclerotic regression (reverse the formation of plaque arterial build up).

In a randomized controlled trial, 223 patients with angiographically proven coronary artery disease (CAD) were given either a fish oil supplement or a placebo for 2 years. [15] The degree of atherosclerotic development was measured in the patients using coronary angiography (a test where radioactive dye is injected in the blood stream so that the diameter of the blood vessels can be viewed and measured) both before and after the treatment. After 2 years of treatment, 41of the 112 patients given the placebo showed evidence of atherosclerotic progression whilst 7 showed mild regression. By comparison, 39 of 111 patients given fish oil showed evidence of atherosclerotic progression, whilst 16 showed mild to moderate regression. The average decrease in diameter of the arteries was somewhat less in the fish oil group. These results indicate that fish oil has at least some beneficial effects on the development of atherosclerosis. Moreover, the fish oil recipients had fewer cardiovascular events throughout the duration of the trial.

Certain types of lipids (fats) in the blood are associated with an increased or decreased risk of heart disease. One such type of lipid known to increase the risk is called triglycerides. A Meta analysis of 17 population-based prospective studies examining risk factors and biomarkers of cardiovascular disease among 46,413 men and 10,864 women found that elevated triglycerides significantly increase the risk of CHD. [16]

Investigations into the effect that fish and fish oil has on serum (the liquid portion of the blood) lipid status has shown that whilst total, LDL “bad” and HDL “good” cholesterol levels are not usually significantly effected, triglyceride levels are significantly lowered. A review of 65 randomized controlled trials found that marine-based Omega 3 fatty acid treatments (2-4 g/day of EPA + DHA) tend to lower triglyceride levels by approx 25-30%. [17]

A review of the evidence by the American Heart Association (AHA) states that research suggests that the ways in which omega-3 fatty acids reduce cardiovascular disease risk may be due to their ability to:

decrease risk for arrhythmias, which can lead to sudden cardiac death.

decrease risk for thrombosis, which can lead to heart attack and stroke.

decrease triglyceride and remnant lipoprotein levels.

decrease rate of growth of the atherosclerotic plaque.

improve endothelial function.

(slightly) lower blood pressure.

reduce inflammatory responses.

The AHA also warns that patients taking more than three grams of these fatty acids from supplements should do so only under a physician’s care, as these intakes could cause excessive bleeding in some people. [18]

Fish Consumption and Cerebrovascular Disease (stroke)

The data regarding fish consumption and stroke is less consistent, as some studies have found no association. [19,20] A prospective study which followed 552 men aged 50 to 69 for 15 years found that compared to men who consumed little fish, those who consumed an average of 20 g per day had a 50% reduced risk of stroke. [21]

The National Health and Nutrition Examination Survey I Epidemiologic Follow-up Study was a longitudinal cohort study of a national sample which followed white and black women and men aged 45 to 74 for 16 years. [22] Frequent fish consumption was only associated with a 15% reduction in stroke among white males, however among black males as well as both white and black females, the incidence of stroke was approx 50% lower for those who consumed fish at least once per week.

The inconsistency seen throughout studies examining the association between fish intake and stroke may be due at least in part to the different types of stroke. Fish and fish oils are known to inhibit blood clotting and slow the development of atherosclerosis; the narrowing and hardening of the arteries due to plaque formation which can cause blockages and lead to ischemic heart attack or ischemic stroke. The word “ischemic” refers to tissue damaged caused a lack of blood supply to an area. If this area is tissue of the heart or brain, the result is a heart attack or ischemic stroke, which can be fatal.

Ischemic stroke can occur due to a significant narrowing and subsequent blockage of the cerebral arteries due to atherosclerosis. When the clot forms at an atherosclerotic plaque, the result is called a “thrombotic stroke”; thrombus meaning clot. A similar type of stroke occurs where a blood clot formed else where in the body (embolism) circulates throughout the blood supply until it gets lodged in the arteries of the brain, called an “embolic stoke”. Due to fish oils ability to inhibit clot formation and inhibit atherosclerosis, it is feasible that fish consumption may reduce the risk of these types of stroke.

The other major type of stroke occurs when an artery in the brain ruptures. The surrounding tissue is damaged due to internal bleeding and the brain tissue further on fails to receive enough blood supply. This is known as a haemorrhagic stoke, the word haemorrhage referring to bleeding caused by the rupture of a blood vessel. This type of stroke generally occurs in much older people, although brain haemorrhages can occur in younger people if caused by some other abnormal damage to the cerebrovascular system. In western populations, haemorrhagic stroke usually accounts for only about 1/3 of all strokes, whereas ischemic strokes are much more common.

It is not likely that consumption of fish would decrease the risk of haemorrhagic stroke; in fact, given its potential to inhibit blood clotting, excessive fish oil consumption through supplements may even increase the risk of death from haemorrhagic stoke in older people because if they do sustain a haemorrhagic stroke, it is vital that the blood be able to clot quickly before irreparable damage occurs to the brain tissue, resulting in disability or death.

This theory has been supported in several studies which have looked at fish intake and the risk of specific types of stroke. The Nurses' Health Study which followed approx 80,000 middle aged women for 14 years found that compared with women who ate fish less than once per month, those with higher intake of fish had a lower risk of total stroke; those who ate fish once per week had a 22% reduced risk, whilst those who ate fish 5 or more times per week had a 52% reduced risk. [23] This association however was highest for thrombotic stroke, whilst no association was found for haemorrhagic stroke.

Similar results were found in the Health Professional Follow-up Study, which followed 43, 671 men aged 40 to 75 for 13 years. [24] Compared with men who consumed fish less than once per month, the risk of ischemic stroke was 43% lower among those who had fish 1 -3 times per month or more. Again, no association was found between fish intake and the risk of haemorrhagic stroke.

Fish and Cancer

Laboratory Studies

There is both epidemiologic and experimental evidence that the long-chain omega-3 fatty acids (FAs), which occur at high levels in some fish oils, exert protective effects against some common cancers, notably those of breast, colon, and, perhaps, prostate.

In vitro laboratory studies have shown that fish oil Omega 3 fatty acids possess multiple mechanisms by which they may exert anti-cancer activity, including suppression of neoplastic transformation (the transformation of normal cells to cancer cells), enhanced apoptosis (the normal cell death that ceases to occur in cancer cells), and the ability to inhibit angiogenesis ( the growth of new blood vessels to a tumour which feeds it with extra nutrients). These effects have been demonstrated specifically in human breast, colon and prostate cancer cells. [25]

Intake of certain poly-unsaturated fatty acids (PUFA’s) have been associated with an increased risk of certain cancers in laboratory experiments, however those found in Fish seem to have a protective effect. Animal experiments have shown that diets containing the Omega 3 fatty acids DHA and EPA can prevent the development and or inhibit the growth of cancers of the breast, [26-29] colon, [30-35] and Lung. [36,37]

Epidemiological Studies

In 1999, the relation between frequency of fish consumption and the risk of certain cancers was analysed by using data from a series of case-control studies conducted in northern Italy between 1983 and 1996. [38] The frequency of fish consumption among 7,990 healthy controls was compared to 181 cases of mouth & throat cancer, 316 cases of esophageal cancer, 745 cases of stomach cancer, 828 cases of colon cancer, 498 cases of rectal cancer, 428 cases of liver cancer, 60 cases of gallbladder cancer, 362 cases of pancreatic cancer, 242 cases of laryngeal cancer, 3,412 cases of breast cancer, 750 cases of endometrial cancer, 719 cases of ovarian cancer, 127 cases of prostate cancer, 431 cases of bladder cancer, 190 cases of kidney cancer, 218 cases of thyroid cancer, 80 cases of Hodgkin disease, 200 cases of non-Hodgkin lymphoma, and 120 cases of multiple myelomas. Although no pattern of cancer risk in relation to fish consumption was observed for cancers of the liver, gallbladder, breast, bladder, kidney, or thyroid or for lymphomas, significantly protective associations were found between frequent fish consumption and the risk of other cancers. Compared to infrequent consumption, those who had frequent servings of fish had the following reduced risks (as a percentage): Mouth and throat: 50%, esophagus: 40%, stomach: 30%, colon: 40%, rectum: 50%, pancreas: 30%, Larynx: 30%, endometrium: 20%, ovaries: 30%, multiple myelomas: 50%.

Whilst these results appear encouraging, many other population based studies which have looked at fish consumption in relation to cancer risk have been largely inconsistent. Whilst several cohort studies have found a slightly reduced risk of breast cancer among frequent consumers of fish, [39-41] the majority have found no significant association. [42-45]

A significant number of case-control studies have also found lower rates of breast cancer among frequent consumers of fish, [46-52] however many others have found little [53-56] or no protective association. [57-64]

Although the results of cohort studies examining the relationship between fish consumption and prostate cancer have also been inconsistent, several large well designed studies have found at least some protective effect. A cohort study which followed 6,272 Swedish men for 30 years also found a protective, inverse association between fish intake and prostate cancer. [65] In this study, men who ate no fish had a two-fold to three-fold higher frequency of prostate cancer than those who ate moderate or high amounts did.

In the large Health Professionals’ Follow-up Study which followed 47,882 men for 12 years, a protective and significant inverse association was found between intake of fish and metastatic prostate cancer. Compared to those who rarely ate fish, there was 44% less prostate cancer among men who had 3 or more servings of fish per week. [66]

Conversely, a large cohort study in the Netherlands which followed 58,279 men for 6 years found no association between the intake of fish or EPA / DHA and the risk of prostate cancer. [67,68] Several other smaller studies have found little [69,70] or no protective association. [71] However at least 5 case-control studies in Europe, Japan and the U.S involving a total of 980 men with prostate cancer compared to 9,445 without (controls) have found that frequent consumers of fish have between a 20 – 60% less prostate cancer than infrequent consumers. [72-77]

Fish Oil and Inflammatory Conditions

EPA and DHA have been shown to decrease inflammation by reducing the release of inflammatory mediators (leukotriene B(4) and interleukin 1 from white blood cells (stimulated neutrophils and monocytes). [78-82]

Due to its anti-inflammatory effect, fish oils have shown some benefit in patients with Rheumatoid arthritis. A meta-analysis of 10 randomized controlled trials involving a total of 395 patients found that fish oil supplementation for 3 months significantly improved the number of tender joints and duration of morning stiffness. [83] It appears however that the levels of fish oils needed to produce this effect in Rheumatoid arthritic patients is above the level considered safe in regards to uncontrolled bleeding. [84] Consequently, such doses should only be taken under medical supervision.

Some studies suggest that fish oil may benefit patients with the inflammatory bowel diseases Crohn’s disease [85] and Ulcerative Colitis, [86-88] however other studies have shown less consistent results. [89-91]

It is widely purported that fish oil supplementation may be beneficial for asthma patients, however studies have been inconsistent. [92]

Fish & The Brain

Cognitive Function

The Omega-3 fatty acid DHA is a primary component of brain cell membranes. [93] High levels of DHA are found in the more metabolically active areas of the brain, including the cerebral cortex, mitochondria, synaptosomes, and synaptic vesicles. [94-96]

Breast milk contains significant concentrations of the Omega 3 fatty acid DHA. Studies have indicated thatbreast-fed children had higher IQ scores and enhanced visual acuity compared with children who received an infant formula that did not contain DHA. Thus it has been suggested that Omega 3 fatty acids may contribute to improved cognitive development. [126, 127]

One study found that higher levels of DHA in the blood of mothers and babies at birth resulted in better cognitive development during infancy compared with those with lower DHA levels.[128] Supplementation with the Omega-3 alpha-linolenic acid during pregnancy however does not appear to have this same effect on subsequent cognitive development of infants. [129] In fact, it has been shown that DHA uniquely promotes new brain cell growth in an area of the brain responsible for long term memory called the hippocampus. Thus it has been suggested that inadequate neurite development due to DHA deficiency may contribute to the cognitive impairment associated with Omega-3 fatty acid deficiency. [130]

One British cohort study which examined the relation between fish consumption during pregnancy and subsequent infant cognitive development followed 7421 new-born children for 18 months. Cognitive development was assessed using adaptations of the MacArthur Communicative Development Inventory (MCDI) at age 15 months and the Denver Developmental Screening Test (DTST) at age 18 months. The results showed that developmental assessment scores consistently increased with greater maternal fish intake during pregnancy. The greatest effect was a 7% higher MCDI comprehension score when mothers ate fish at least once a week, compared with those eating no fish. MCDI social activity scores were higher in the children whose mothers consumed any fish throughout pregnancy, compared to those whose mothers consumed no fish at all. It was also found that the child's fish intake was independently associated with higher neurodevelopmental scores. Fish intake during pregnancy also was associated with increased levels of mercury in umbilical cord tissue; however it was not apparent what effect if any this may have had on the subsequent health of the child.

The authors of this study concluded that “Moderate intake of non-contaminated fish during pregnancy may enhance the child's early development of language and communication skills.” [131]

Senile Dementia

A cohort study which followed 246 elderly men and women for 4 years found that those with higher blood levels of stearic acid (a type of saturated fatty acid) and Omega-6 fatty acids had a significantly higher incidence of cognitive decline. Those with higher blood levels of Omega-3 fatty acids however had a 41% reduction in cognitive decline. [132]

In the Rotterdam Study, assessed thefood intake of 5,386 non-demented men and women over 55 and were followed for more than 2 years. The results found that higher intakes of cholesterol and saturated fat markedly increased the risk of dementia however fish consumption showed an inverse association. The highest consumers of fish had 60% less total dementia, and specifically 70% less Alzheimer’s Disease compared with non fish consumers. [134]

The Personnes Agees QUID cohort, an epidemiological study examining cognitive and functional aging among elderly (>65 years) people in South-western France, followed and analysed the dietary intakes of 2,950 initially non-demented elderly citizens for 8 years. During this time, 170 new cases of dementia occurred, including 135 cases of Alzheimer's disease. The results showed that the incidence of dementia was 44% lower among those who ate fish or seafood at least once a week. [135]

More recently, the Chicago Health and Aging Project, a population-based study aimed at identifying risk factors for Alzheimer disease, surveyed and administered cognitive testing among 6,158 men and women aged 65 – 94. Re-evaluation of the surviving participants several years later revealed that 131 persons had been diagnosed with Alzheimer ’s Disease. Analysis of their dietary intakes found that participants who consumed 1 or more servings of fish per week had 60% less risk of Alzheimer’s Disease than did persons who reported eating fish rarely or never, whereas those who ate 2 or more serves per week had a 70% reduced risk. [136]

Depression

A study which found that low DHA levels predict low concentrations of a marker of brain serotonin (a neurotransmitter associated with depression) turnover, [137] led researchers to discover that data from ecologic studies across 9 different countries shows an inverse association between fish consumption and the annual national prevalence of major depression.

Studies have shown that patients with depression have significantly lower blood cell levels of DHA compared to non-depressed people. Moreover, the severity of depression among depressed patients was even greater in those with lower Omega-3 intakes and DHA blood cell concentrations. [138] Further investigations also found that among moderately to severely depressed patients, there was a significant correlation between the degree of symptom severity and the ratio of Omega 6 fatty acids and the Omega 3 fatty acids; that is, patients with lower Omega 3 levels were more severely depressed. [139]

A recent study in China found that blood levels of EPA were significantly lower among 100 patients who had attempted suicide compared with 100 healthy controls. [140] Several other studies have confirmed that depressed patients generally have significantly lower levels of Omega-3 fatty acids than non-depressed people do. [141-143]

To test whether fish oil supplementation can improve symptoms of depression; a recent randomized double blind trial in New Zealand gave 59 depressed patients either 8g of fish oil or olive oil as a placebo for 12 weeks. The results showed that there was no significant difference in symptoms between the fish oil group and the placebo, however a significant improvement occurred after 12 weeks in both groups and continued until the end of the trial. This was a poorly designed study because it may well be possible that olive oil may also have a beneficial effect on depression symptoms, consequently a non-pharmacologically active substance (a dummy pill) should be used as a placebo in further studies. [144]

Other trials have found that high doses of fish oil have offered some relief of depressive symptoms in patients with bi-polar disorder (manic depression) [145] and borderline personality disorder [146] however these results are very preliminary. Further studies are needed before any recommendations to give potentially dangerous large doses of fish oil to depression patients should be made. At this point, it does not appear evident whether patients with low Omega-3 fish intake are more susceptible to depression, or whether patients with depression have some kind of inability to metabolise Omage-3 fatty acids properly. Either way, there is at least some good evidence that moderate fish consumption may be a worthwhile part of dietary interventions in depressed patients.

Mercury

In recent years, an increasing concern regarding the potential danger of mercury levels found fish due to contamination from coal-fired power plants. [147,148] This has lead to the recommendations that pregnant women do not consume certain types of fish. The good news however that many Omega-3 rich fatty fish such as fresh salmon do not harmful levels of mercury. Fish which tend to accumulate potentially harmful levels of mercury include large predatory fish such as king mackerel, tilefish, shark, or sword-fish. In fact, the mercury content of some of these larger predatory fish is up to 100 times that of fish considered safe such as salmon, whiting, trout, sardines, shrimp, hake and anchovies. A full list of mercury levels found in various fish species including those considered safe and those considered unsafe can be found on the U.S. Food and Drug Administration’s (FDA) website at: http://vm.cfsan.fda.gov/~frf/sea-mehg.html

[59] Kato I, Miura S, Kasumi F, et al. A case-control study of breast cancer among Japanese women: with special reference to family history and reproductive and dietary factors. Breast Cancer Res Treat 1992;24:51–9.[Medline]